This invention relates to tunable bandpass filter systems and filtering methods, and particularly to the use of a tunable bandpass filter whose bandwidth is adjusted in response to the fundamental frequency of an input signal to a test device to measure audio frequency distortion by determining the amplitude of the harmonics of the fundamental frequency produced by the test device.
Audio signal distortion measurements are often made by applying to a device whose distortion is to be measured a sinusoidal input signal whose frequency is known and determining the amplitude of one or more harmonics of that frequency at the output of the test device. In one distortion measurement the amplitude of a selected harmonic is measured while the frequency of the input signal is swept over a predetermined range. In another distortion measurement the amplitudes of several harmonics of the input signal are measured while the frequency of the input signal is held constant.
A bandpass filter is ordinarily used to make the distortion measurements described above. Bandpass filters generally fall into one of two broad categories: constant bandwidth bandpass filters and constant Q ("quality factor" or "percentage bandwidth") bandpass filters. In the case of a constant bandwidth filter the bandwidth of the filter can be adjusted, but once adjusted, it stays the same regardless of the center frequency, that is, the measurement frequency, to which the filter is tuned. In the case of a constant Q filter, the Q, that is, the ratio of the center frequency of the filter to its bandwidth, can be adjusted and once the Q is adjusted, the Q stays the same regardless of the center frequency to which the filter is tuned.
Where the input signal frequency is to be varied while the amplitude of a selected harmonic is measured, a constant Q filter is ordinarily used. Since the bandwidth of a bandpass filter must be narrow to discriminate between harmonics at low frequencies and since the frequency range to be measured extends over three decades, use of a constant bandwidth filter would make tuning of the center frequency to a selected harmonic of the input signal, or to the harmonic of an input signal whose frequency is unstable, increasingly difficult as the center frequency increases. However, the bandwidth of a constant Q filter increases as its center frequency increases thereby keeping the ability to tune the filter to a given harmonic the same at all center frequencies.
Where the frequency of the input signal is to be held constant while the amplitudes of several harmonics of the input signal are measured, a constant bandwidth filter is ordinarily used. Since harmonics of the input signal are equally spaced, use of a constant Q filter would cause the bandwidth of the filter to overlap more than one harmonic as the center frequency increases, resulting in inaccurate amplitude measurements. However, holding the bandwidth of the filter constant, and less than or equal to the separation of the harmonics, prevents this problem from occurring.
Spectrum analyzers and other signal distortion measurement devices are known which employ either a constant bandwidth or a constant Q bandpass filter, but neither is entirely adequate for the reasons explained above. However, it has been discovered that by constructing a bandpass filter system wherein the filter bandwidth is a function of the fundamental frequency, regardless of whether the filter is of the constant bandwidth or constant Q type, the advantages of both types of previously known filters can be obtained in a single apparatus and method.
In addition to known spectrum analyzers and distortion measurement devices, devices are known which vary their bandwidth to accommodate the spectrum of an input signal, as disclosed by Yoneyama U.S. Pat. No. 3,742,395, and which provide a low-pass filter whose upper frequency cut off tracks the fundamental frequency of an input signal so as to provide a constant amplitude output signal, as disclosed in Neuman U.S. Pat. No. 3,644,847. However, none of these devices provides a solution to the problems discussed above.